What Is Synastry? How Astrology Compares Two Charts

Synastry is relationship astrology: the practice of laying two birth charts over each other to see how two people's planets interact. Where compatibility-by-sign is the headline, synastry is the full article, the actual mechanics of why two specific people click, clash, or can't look away. Compare two charts now: synastry calculator.

How it works

Each person has a full chart, Sun, Moon, Venus, Mars, the rest, scattered across the zodiac. Synastry looks at the angles between your planets and theirs: when your Venus sits close to their Mars, when your Moon meets their Saturn, when your Sun touches their Ascendant. These contacts, called aspects, are where the relationship's chemistry and friction actually live. A trine between two planets flows; a square grates; a conjunction fuses. The pattern of contacts is the relationship's fingerprint.

The contacts that matter most

A few carry outsized weight. Sun-Moon between two charts is one of the classic markers of lasting compatibility, deep mutual understanding. Venus-Mars is the attraction engine, desire and romance. Moon-Moon shows emotional rapport. Saturn contacts bring commitment and weight, sometimes too much. And contacts to the Ascendant or the angles create the instant "I feel like I've known you forever" recognition. A strong relationship usually has several of these, not just one.

Synastry vs composite

Two different tools, often confused. Synastry compares the two charts as separate things, your planets and theirs. A composite chart merges both into a single third chart representing the relationship itself as an entity. Synastry answers "how do we affect each other"; composite answers "what is this relationship, as its own thing." Serious relationship astrology uses both.

Frequently asked questions

What is synastry in astrology?

The comparison of two birth charts to see how each person's planets interact, the contacts between them reveal the chemistry, friction, and dynamics of the relationship.

What is the most important aspect in synastry?

Sun-Moon contacts are among the strongest markers of lasting compatibility, while Venus-Mars drives attraction. Most strong relationships show several significant contacts rather than relying on one.

Do you need birth times for synastry?

Birth times sharpen it considerably, they're required for the Moon's exact degree and for the Ascendant and houses. Without times you can still compare Sun, Venus, and Mars, which carries real signal.